Exemplary embodiments of the present invention relate to a memory device including a variable resistance element.
A dynamic random access memory (DRAM) is a widely used semiconductor memory. However, there are limitations with respect to scalability and capacitance of a DRAM. To address such concerns, different types of memory devices are being developed. Examples include a magnetoresistive random access memory (MRAM) using tunneling magneto-resistance, and a phase-change random access memory (PRAM) using a resistance difference of a phase-change element.
An MRAM is a nonvolatile memory device using a magneto-resistance change that depends on magnetization directions of two ferromagnetic layers constituting a magnetic tunnel junction (MTJ). The MTJ has a stacked structure including a ferromagnetic layer, an insulation layer, and another ferromagnetic layer. At this time, one of the two ferromagnetic layers is a pinned layer (PL) whose magnetization direction is pinned, and the other is a free layer (FL) whose magnetization direction is changed by a current passing therethrough. When electrons tunneling through the first ferromagnetic layer pass through the insulation layer used as a tunneling barrier, the tunneling probability changes depending on the magnetization direction of the second ferromagnetic layer. That is, the tunneling probability is the highest when the magnetization directions of the two ferromagnetic layers are parallel to each other (that is, in the same direction) and is the lowest when the magnetization directions of the two ferromagnetic layers are anti-parallel to each other (that is, in the opposite directions). Therefore, stored data can be read by using a difference in current in two cases.
An MRAM relies on a spin transfer torque (STT) phenomenon in writing data to a memory cell. In a STT phenomenon, a spin-polarized current is transferred as an angular momentum of a ferromagnetic material due to a change of an angular momentum instantly generated when the spin-polarized current passes through the ferromagnetic material. That is, when a high-density current having a polarized spin direction is applied to a ferromagnetic material, data is written due to a phenomenon where a spin direction of a current is aligned (for example, changed) when a magnetization direction of a ferromagnetic material is not equal to a spin direction of a current.
In the MTJ used in a semiconductor memory, when electrons flow from a pinned layer to a free layer, a torque is generated to align the magnetization direction of the free layer by a flow of electrons whose spin directions are aligned in the magnetization direction of the pinned layer. Consequently, the magnetization direction of the free layer is parallel to the magnetization direction of the pinned layer at a certain current or more. On the other hand, when electrons flow from a free layer to a pinned layer, spin accumulation phenomenon occurs at an interface between the pinned layer and the free layer, so that the magnetization direction of the free layer is anti-parallel to the magnetization direction of the pinned layer. Therefore, data can be written in the magnetization direction of the free layer.
Meanwhile, a PRAM uses a special thin film, called chalcogenide. A chalcogenide alloy has a feature that its resistance increases at an amorphous phase and decreases at a crystal phase. Data is written by the control of the two phases.
A PRAM includes a chalcogenide compound and a resistance element, and an amorphous phase and a crystal phase are changed depending on a voltage applied thereto. A change from an amorphous phase to a crystal phase is achieved by applying a voltage for a certain time. A current flows between a chalcogenide compound and a resistance element. When the current continuously flows, Joule heat is generated in the resistance element. Thus, an atomic structure is reorganized and changes to the crystal change. On the other hand, a change from a crystal phase to an amorphous phase is achieved by a rapid cooling from a high temperature. To this end, a voltage is applied for a short time to generate Joule heat, and an applied voltage is rapidly lowered from the point of time when Joule heat is generated. Therefore, the phase change between the amorphous phase and the crystal phase is controlled by a time width of an applied pulse voltage.